Cavers Daniel Caron and Luc Le Blanc found the new chamber, part of the Saint-Léonard cavern underneath Parc Pie-XII, at the end of October, though they had long suspected it existed. (They prefer to be called cavers instead of spelunkers.)

The pair found it by digging through a soft layer of limestone. When they broke through the rock, they could see through into what Le Blanc called "the void beyond."

They toured media through the chamber on Friday. The head of the Quebec Speleology Society confirmed the public will eventually be able to access the new part of the cave, but the group wants to make sure it can protect the stalactites and stalagmites first.

Here's a closer look at the enormous chamber.

A young cave

According to the City of Montreal, the cave itself is "young," somewhere around 10,000 years old, though Caron and Le Blanc suggest it's closer to 15,000 years or older. The main part of the Saint-Léonard cavern is 35 metres long and drops down about eight metres. ​

(Graham Hughes/Canadian Press)

A major discovery

"It's very significant, because in a caver's life, especially living in Quebec, this is pretty incredible", said Le Blanc. Below, a ladder peeks out of a tunnel on the way to the newly-discovered chamber.

(Graham Hughes/Canadian Press)

Through the limestone

In past years, Le Blanc and Caron were digging lower down a rock wall to try to find a point into the chamber. This year, they dug at a higher point on the wall and opened what Le Blanc described as a "window" into the void. The discovered it one day, and then returned the following day with a ladder because they knew they were coming into the chamber from above.

(Charles Menard/Radio-Canada)

Perfectly vertical walls

Le Blanc and Caron used an inflatable canoe to explore the chamber.

"We did about a hundred metres. The walls are perfectly vertical," said Le Blanc. "It's just beautiful."

(Quebec Speleology Society)

Pushed apart by glaciers

"The formation itself, of this cave, is pretty special," said Le Blanc. "The walls just opened through the pressure of the glacier above about 15,000 years ago or perhaps more," he said. "It's a mechanical process through a glacier, it's been called glacial tectonism."

In the photo below, the caver closer to the bottom of the photo is standing next to an outcropping of rock. That outcropping matches a notch on the other side of the passage, which indicates the rock was pushed apart by the pressure of the glacier.

(Quebec Speleology Society)

Stalactites

Stalactites hang down from the ceiling of cave. They're formed by water dripping down rock over time, leading to calcite formations that get longer and thicker over time as the calcite builds up.

You can tell them apart from stalagmites because stalagmites might hang down from the ceiling but they actually don't. In some caves, the calcite builds up so much that stalactites and stalagmites fuse into one cool column of dripping wet rock.

(Charles Menard/Radio-Canada) (Société québécoise de spéléologie)

At the aquifer

The water in the new cavern is part of the Montreal aquifer, according to Le Blanc. An aquifer is basically an underground layer of permeable rock that holds water. Think of it like a rock bank that holds water reserves until you withdraw them to use.

(Graham Hughes/Canadian Press)

Layers of rock

The cave isn't warm — the City of Montreal says the average temperature is about 5 C.

(Graham Hughes/Canadian Press)

The cave's relative humidity registers at nearly 100 per cent, which caused a few problems for Martin Hazel, the Radio-Canada cameraperson. He had to pause during the cave tour to clean the humidity off his lens.

Paddling through

"It's five metres of water underneath," estimates Le Blanc of the water in the chamber. "And it keeps going, we haven't reached the end yet."

(Quebec Speleology Society)

A major discovery

This is a major discovery we made, this doesn't happen many times in a lifetime," said Le Blanc.

(Graham Hughes/Canadian Press)

With files from Jaela Bernstien, Charles Ménard and Jean-Sébastien Cloutier

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